1. Field of the Invention
The present invention pertains to lubrication systems for lubricating the bearings of bearing equipped items. More specifically, the present invention pertains to lubrication systems in which an oil mist is formed by combining air and oil and then distributed in the form of a dry oil mist to the bearings to be lubricated. More specifically, the present invention pertains to an oil mist gauge for determining the density of oil mist flowing through an oil mist lubrication system.
2. Description of the Prior Art
For many years bearings were lubricated by a "one shot" application of grease and/or oil to a grease or oil fitting with a grease gun or oil can. Although attempts were made to apply the grease or oil at periodic frequencies, many times too much oil and/or grease was used and, at other times, not enough oil and/or grease was provided for lubrication. For this reason, lubrication systems which apply the lubricant at timed periodic intervals or on a continuous basis were developed. U.S. Pat. No. 4,445,168 discloses a computer controlled lubrication system in which individual "shots" of lubricant are periodically delivered based on either a time cycle or a machine stroke cycle. U.S. Pat. No. 4,527,661 utilizes what is referred to in the industry as an "air-oil lubrication system" in which separate oil and air streams are brought to and combined by a mixing device, i.e. an atomizer, at a point immediately adjacent to the bearing being lubricated. These systems require two sets of piping (one for oil and one for air) and individual mixing devices at each point of lubrication.
In more recent years, oil mist lubrication systems have been developed to provide continuous, more effective lubrication to anti-friction bearings of rotating equipment such as centrifugal pumps, electric motors, speed turbines, gear boxes, blowers and fans. An oil mist lubrication system typically comprises an oil mist generator in which a compressed air stream, in turbulent flow, is combined with a liquid lubricant to create a fine mist of oil particles suspended in an air stream. These oil particles are typically one to five microns in diameter. The oil mist is transported through a piping system and delivered to the bearing housings of rotating equipment. The oil mist continuously lubricates the bearings of the equipment and maintains a slight positive pressure in the bearing housing to reduce contamination from outside sources. When oil mist is generated by such a system, the oil is atomized into very fine particles, typically one to five microns in diameter, so that the oil mist will remain stable and can be transferred relatively long distances with minimum wetting out on the walls of the pipe in which it is being conveyed. These fine particles, referred to as "dry oil mist", must be converted into larger particles, referred to as "wet oil mist", in order to wet out on the metal surfaces of the equipment bearings being lubricated. This is accomplished by passing the dry mist through a specially designed restriction orifice known as a "reclassifier". The reclassifier induces turbulence in the stream to combine small particles into larger ones before the mist (wet oil mist) enters the equipment bearing housing. These reclassifiers serve the additional purpose of metering the amount of lubricant to each bearing to avoid over or under lubrication. Selection of the correct reclassifier for each application point in the system is based upon an understanding of the exact bearing configuration for each piece of equipment to be lubricated. Such a system is described in U.S. Pat. No. 5,125,480.
U.S. Pat. No. 5,318,152 discloses an even more advanced oil mist lubrication system in which oil mist from an oil mist generator is distributed through a distribution assembly which includes one or more reclassifiers for converting the dry oil mist to a wet oil mist just prior to application to be bearing to be lubricated. This system also provides collection means into which excess oil and oil mist may flow and accumulate after lubrication of bearings. The excess oil mist and, in some cases, the excess oil collected may be returned for recycling and reuse. A demisting filter may be provided for separating the returned excess oil mist into oil and oil-free air, the oil accumulating for reuse and the oil-free air being vented to the atmosphere.
To assure that an oil mist lubrication system is supplying sufficient lubricating oil, the quantity of oil in the oil mist of an operating oil mist system must be determined. If the quantity of oil is too low, the bearings of the system may not be sufficiently lubricated. If the quantity of oil in the oil mist is too high, too much lubricating fluid will be wasted. A waste of oil, such as mineral oil, could be expensive and a waste of synthetic oil, typically more costly than mineral oil, could be very expensive. Thus, it is important to measure or monitor the density of oil in the oil mist of an oil mist system.
In prior procedures, the quantity of oil in the oil mist of an operating oil mist system (referred to in the industry as the mist density) have been difficult to carry out and do not yield good results. The procedure most commonly followed is a "consumption test". In such a test, oil usage over a set period of time is measured and based on the SCFM (system cubic feet per minute) of the system, as defined by totaling the flow of all reclassifiers, the mist density is supposedly measured. Such a test not only takes a long period of time, in most cases over 24 hours, but is not technically accurate. A typical consumption test procedure follows:
1. The consumption test involves measuring the change in the oil level as shown on the level gauge of a central oil mist generator reservoir. PA1 2. The automatic oil fill option on the generator is turned off during the test which runs over a 24 hour period. Automated drain legs are also deactivated. PA1 3. The change in level on the level gauge is converted to a volume measure by applying the known cross-sectional area of the generator reservoir to the level change, thus determining the amount of oil sent out of the unit over the duration of the test. PA1 4. The input air volume of the system is assumed to be equal to the sum of the rated flow of all reclassifiers in the system. PA1 5. The flow rate through each reclassifier is assumed to be the design value for a system operating at 20 inches of water column pressure; 501 .perspectiveto.0.09 SCFM, 502 .perspectiveto.0.18 SCFM, etc. PA1 6. A physical count of the reclassifiers is made, a difficult and time consuming task on large mist systems. PA1 7. The system is set to operate at 20 inches of water column pressure. PA1 8. Mist density or oil/air ratio is then calculated by taking the cubic inches of oil sent out of the generator over the test period and dividing this figure by the presumed air consumption of the system. PA1 9. The figure is normalized to an hourly rate so that the result is presented as "cubic inches of oil per hour per SCFM".
Such a procedure can be demonstrated as being obviously inaccurate by considering two mist systems which are the same (head size, flow rate, number of lubrication points, etc.) except that with one the mist header is sloped away from the generator while in the other the header is sloped toward the generator. Assume that in such a consumption test of both systems the reduction in oil level of the two generators was the same over the 24 hour test period. The test calculations as defined in the above procedure would then show that oil consumption and therefore, implied oil mist density, was the same for each system because the outputs of oil from each reservoir were identical. One can easily see, however, that the mist density of the two systems is not identical. The system with the header pipe sloped toward the generator must produce a much denser oil mist to achieve the same reduction in oil level in the reservoir since all of the oil mist coalescing in the header flows back to the generator reservoir. The system with the header pipe sloped away from the generator needs only to produce a much leaner oil mist to achieve the same measured consumption rate. The reason for this is the oil that coalesces in the header does not flow back to the generator reservoir. In measuring such a system in which the header pipe slopes back toward the generator, it may be assumed that 25% of the oil mist coalesced into liquid and collected in the header. This quantity of coalesced oil has a significant effect on measuring the oil consumption. In the assumed specific case of 25% return in a system where the header slopes toward the generator, the true mist density produced by the mist generating head must be 0.87 cubic inches of oil per hour per SCFM to achieve a net output of oil equal to 0.65, requiring a 33% richer and denser mist. Thus, measuring mist density by the traditional consumption test is inaccurate as the results are influenced by the header sloping.
It has been determined that mist flow to a bearing should be based on a target mist density of 0.65 cubic inches of oil per hour per SCFM. There is no need to be above this level and in fact many believe that the oil/air ratio can be less than 0.65. A more accurate method and apparatus for measuring oil mist density (oil/air ratio) is very much needed. A device for quantitatively and absolutely measuring the amount of oil in the oil mist stream, independent of header slope, is needed.